This invention relates to a spark gap device for a lightning arrestor suited for protection of a direct current transmission line, and more particularly to a spark gap device for a lightning arrestor wherein reigniting spark gaps are provided at positions different from that at which a spark starting gap is provided.
The more increased the transmission voltage is, the more important the role of the lightning arrestor connected to the transmission line becomes. Generally, the surge energy produced in the transmission line is substantially proportional to the second power of the transmission voltage. The actual circumstance, however, is that the performance of the lightning arrestor is not elevated to such an extent as is sufficient to resist the increasing transmission voltage. The electrostatic capacity of a cable used for the direct current transmission is as large as approximately 30 times that of an overhead wire used for the alternating current transmission. Accordingly, the amount of surge energy produced in a long direct current transmission line is extremely large and this large amount of surge energy should not be treated by the lightning arrestor but the following current interruption, namely, the direct current interruption should also be performed by the lightning arrestor. Accordingly, with the prior art lightning arrestor it is difficult to protect the alternating current transmission line of a voltage higher than, for example, 500 kv or long-distance direct current transmission line. Generally, the lightning arrestor is connected to the direct current transmission line as shown in FIG. 1A. That is to say, the output from a transformer Tr is transmitted to a power receiving end through an AC-DC converter Rf, smoothing reactor L and DC transmission line DCL. A first lightning arrestor LA1 is connected between the junction point between the smoothing reactor L and the transmission line DCL and ground, and a second lightning arrestor LA2 is connected between the junction point between the smoothing reactor L and the AC-DC converter Rf and ground. The waveforms of surge currents flowing in said lightning arrestor when the arrestors are in their normal operations are shown in, for example, FIGS. 1B and 1C. The peak value ip of the surge current flowing in the lightning arrestor LA1 amounts to as large a value as 1000 amperes to 3000 amperes and, in the case of the long-distance transmission line, the lasting time t of the surge current thereof is as long as, for example, 5 milliseconds due to the lumped capacity. On the other hand, since the surge current flowing in the arrestor LA2 flows therein through the reactor L, the peak value ip' thereof becomes smaller than said peak value ip as shown in FIG. 1C. In this case, however, the lasting time t' thereof sometimes is as long as 10 milliseconds. The arrestors LA1 and LA2 are respectively comprised of a plurality of series-connected spark gap devices and a non-linear resistance element serially connected to said devices, which are received within an airtight chamber into which, for example, a nitrogen gas is charged or sealed. The prior art spark gap device is constructed such that a pair of main electrodes are disposed within an arc extinguishing chamber constituted by two arc extinguishing plates so as to form a starting spark gap, and an arc produced at the starting spark gap is driven by the magnetic field produced due to the current flowing in a coil (not shown) connected in series to said starting spark gap, or the magnetic field produced from a permanent magnet not shown so that the arc is extended within the chamber. The arc thus extended is rapidly cooled when contacted with the inner walls of the arc extinguishing chamber, to cause the arc voltage to be increased, so that the arc current, accordingly, the following current is interrupted. For the purpose of performing the cooling effectively, the thickness of the chamber portion (i.e., a distance between the inner walls of the two arc extinguishing plates) where the arc extended up to a final position is passed is rendered smaller than that of the chamber portion where the arc in the course of being extended is driven toward the final position. The spark gap device having the foregoing construction is called a current limiting-spark gap device and has the following drawbacks.
1. Since the arc current is confined for approximately 5 milliseconds within that chamber portion having said smaller thickness which corresponds to said final position of the arc, the corresponding arc extinguishing plate portion is likely to be carbonized or fused. In such a case, it is impossible to increase the arc voltage level and therefore to perform the following current or direct current interruption.
2. Difficulty is presented in decreasing the switching surge voltage level to below a limiting level. Generally, the switching surge voltage level is required to be limited to less than a prescribed level. As above described, however, even in the case where the switching surge voltage is treated, the arc is quickly driven up to the final position. By adding the IR voltage drop of said non-linear resistance element of the arrestor to said arc voltage, the limiting level of the switching surge voltage is determined. When the switching surge current is increased in amount, said IR voltage drop is also increased, so that it is necessary to reduce the arc voltage in level. But the arc voltage level can not be reduced to a desired extent.
3. Long flow of the surge current causes the arc extinguishing chamber to be deteriorated or damaged and an increase in the arc voltage level within a small length of time fails to decrease the limiting level of the surge voltage, so that it is difficult to sufficiently absorb the switching surge energy.
For the purpose of eliminating the above-enumerated drawbacks a spark gap device having the following construction has been proposed. The spark gap device is so constructed that while the surge current remains to have a value greater than prescribed, reignition is repeatedly caused between two main electrodes; and when the surge current value is reduced to a value smaller than prescribed, the arc is extended up to a final position and interrupted. In more detail, the main electrodes are respectively provided with a groove extending from a prescribed position of each main electrode to the starting spark gap. When the foot of an arc produced in the starting spark gap has arrived at said prescribed position, an ionized heated gas occurring at the prescribed position is supplied to the starting spark gap through said groove. In this case, during the period in which the surge current value remains greater than prescribed, reignition is repeatedly caused in said starting spark gap, whereas when said surge current value is reduced to a value smaller than prescribed, the ionized heated gas supplied to the starting spark gap does not cause the occurrence therein of reignition. Accordingly, the arc so limited as to have a small current value is extended up to a prescribed position and interrupted. The spark gap device having the foregoing construction is called a reigniting spark gap device, which is capable of eliminating the previously mentioned drawbacks (1), (2) and (3).
Since, however, reignition is caused in an interspace between the main electrodes and yet caused in the interspace portion where the starting spark is caused or in the proximity thereof, the insulation recovery between the main electrodes after the following current interruption is delayed, and further the metal material constituting the main electrode portion where the starting spark is caused is likely to be damaged and in such cases the starting spark characteristics are undesirably varied. Therefore, the use of the arrestor having the above-constructed spark gap device for protection of the direct current transmission line can not be said to sufficiently serve the purpose.
Accordingly, the object of the invention is to provide a spark gap device for a lightning arrestor capable of eliminating the drawbacks encountered with the aforesaid current limiting-spark gap device for a lightning arrestor and the aforesaid reigniting-spark gap device for a lightning arrestor.